This invention relates to power window controls for automotive vehicles and, more particularly, to a control circuit for such devices.
For safety, the motor of window lifter installations in motor vehicles must be switched off immediately, or its direction of rotation has to be reversed, when the window meets an obstacle. In known versions this is ensured by a current supervising stage which responds as soon as the motor current exceeds a given threshold value determined experimentally. On the other hand, the motor must be able to bring a tight window home into the end position. The torque reserve necessary for it in most installations is greater than the admissible torque for safety considerations. Known circuit arrangements with a static evaluation of the motor current proportional to the torque of the motor are not successful in practice, because these conflicting requirements could not be complied with.
The invention is based on the problem to develop a circuit arrangement for such an actuating drive, by which the danger of injury is effectively avoided, but in spite of this the movable element is driven with a sufficient torque in particular cases. This problem is solved by the features of the present invention.
The invention is thereby based on the knowledge that a continuous change in the torque and in the current may be measured, when the movable element enters the end position, thus when for instance a window enters the upper window guidance, whereas in contrast thereto a change in torque or in the current is very rapidly effected, when the movable element meets an obstacle. While in known installations the current supervising stage alone responds to the instantaneous value of the motor current, in a circuit arrangement according to the invention a dynamic current control is effected. Thereby it is simultaneously achieved that the current supervising stage does not respond, when the movable element is continuously tight, because a rapid change in current is not effected. Thus dimensioning of the circuit arrangement is less critical and in this case the maximum torque of the motor can be used to drive the movable element.
According to an advantageous improvement of the invention it is provided that the current supervising stage responds in addition--as in itself known--to a given static current threshold value. The combination of the static and dynamic current control improves the operating reliability and above all is more accident-proof, because the motor switches off too and reverses its direction of rotation respectively, when during a gradual current increase to a value near the short-circuit current a current increase releasing the response of the current supervising stage is no longer possible. It became apparent that it is of advantage to drive the movable element by a motor the maximum torque and the short-circuit current of which respectively considerably exceeds the value which under the most unfavorable operating conditions is necessary to adjust the movable element, because in this case, where the dimensions of the dynamic response threshold of the current supervising stage are adapted to a relatively rapid current increase in time, the current supervising stage reacts rapidly on extremely high values, also after a gradual current increase.